If you install the CMon the installer will ask you for the IP address of the CMon server.
Three options (3rd is recommended) are possible:
Only internal clients connect to CMon:
Configure the local IP of the server.
Clients from outside your LAN will not be able to access the CMon.
Only outside clients connect to CMon:
Configure the external IP.
Internal clients will not be able to connect to the CMon.
Outside AND inside clients (recommended method)
Use an URL that is registered in your DNS, e.g. cmon.yourdomain.com. During the installation of the CMon enter this URL, do NOT enter an IP address. On the client enter this URL in the default settings => tab "CMon Server".
If you want to start the CMon GUI on the CMon server itself and you configured an URL (not IP), then you have to add the following line to your local hosts file
cmon.yourdomain.com <local IP address of CMon server>